Cation exchange media, such as zeolites and sulfonated styrene-divinylbenzene co-polymers, are commonly used to remove cationic impurities from solutions. One of their commercial uses is in water softening, where hard water is passed through a bed of cation exchange media to remove the divalent cations causing hardness in water such as calcium (Ca+2) and magnesium (Mg+2) cations (also referred to as “hardness ions”) which can cause problems such as scale formation, mineral stains in laundry, spotting of dishes, and reduced effectiveness of detergents. When hard water is passed through the cation exchange medium, the positively charged hardness ions bind to anionic sites on the cation exchange medium and typically displace monovalent cations (also referred to as “monocations”) such as hydrogen (H+), potassium (K+), or sodium (Na+). Thus, hard water is softened by replacing the hardness cations (e.g. calcium (Ca++) and magnesium (Mg++) in the water with monovalent cations (e.g., hydrogen (H+), potassium (K+), sodium (Na+)). Periodically, all of the cation exchange sites on the cation exchange medium become loaded or saturated with hardness cations, and the cation exchange medium must be regenerated. Regeneration of the cation exchange involves flushing the cation exchange medium with a more concentrated solution of a monovalent salt (e.g., NaCl) or strong acid (e.g., HCl), which displaces the hardness cations from the cation exchange medium and replaces monovalent cations on the cation exchange sites so the cation exchange medium can resume its softening service cycle (e.g., replacing the hardness cations (e.g. calcium (Ca+2) and magnesium (Mg+2)) in the water with monovalent cations (e.g., hydrogen (H+), potassium (K+), sodium (Na+)).
Potassium chloride (KCl) and sodium chloride (NaCl) are most commonly used as regenerants for residential water softeners using cation exchange media.
The chloride anion (Cl−) plays no direct role in the water softening cation exchange process. The chloride anion (Cl−) is only present as a “spectator anion” because any source of positively charged sodium (Na+) or potassium (K+) cations must include some negatively charged counter anion to permit charge neutrality. An idealized description of a water softening process involves replacing one calcium ion (Ca2+) in water with two sodium ions (Na+) donated by a cation-exchange resin.
While it may be commercially impractical to eliminate the need for sodium (Na+) or (K+) potassium cations in standard, residential, cation exchange water softening, in principle it may be possible to seek alternatives to the chloride (Cl−) anion in the water softener salt, i.e. to use some non-chloride sodium or potassium salt as the regenerant. Accordingly, it may be desirable to provide a regenerant for a cation exchange media for a water softener having reduced chloride. Unfortunately, there can be one or more challenges to this. For example, it is desirable that the salt regenerant be non-toxic, highly water soluble, and the anion of the salt regenerant should not interfere substantially with the ion exchange process.